1. Field of the Invention
The present invention relates to a hybridoma that produces monoclonal antibodies specific to a cell surface antigen associated with multidrug resistance in human cells, and uses of such antibodies and their fragments or recombinant derivatives.
2. Description of the Related Art
Many human cancers express intrinsically or develop spontaneously resistance to several classes of anticancer drugs, each with a different structure and different mechanism of action. This phenomenon, which can be mimicked in cultured mammalian cells selected for resistance to certain plant alkaloids or antitumor antibiotics such as colchicine, vinblastine and doxorubicin (former generic name is adriamycin), is generally referred to as multidrug resistance ("MDR")..sup.1 This MDR phenotype presents a major obstacle to successful cancer chemotherapy in human patients.
MDR, in most cases, appears to result from decreased intracellular accumulation of drug as the result of increased drug efflux related to alterations in a plasma membrane mechanism. When mutant cell lines expressing MDR are isolated, they are seen to express an ATP-dependent pump mechanism located in the plasma membrane that keeps the intracellular accumulation of an anti-cancer drug low. This mechanism consists of active extrusion of the drug, which had originally entered through the plasma membrane.
The gene encoding this pump system, sometimes referred to as the multidrug transporter, has been cloned from cultured human cells by Roninson et al. (see reference 12 below), and is generally referred to as mdr1 or MDR1..sup.1 This gene is expressed in several classes of normal tissues.sup.1, but physiological substrates transported for the mdr1 gene product in these tissues have not been identified.
The protein product of the mdr1 gene, generally known as P-glycoprotein ("P-170", "Pgp"), is a 170 kDa trans-plasma membrane protein that constitutes the aforementioned energy-dependent efflux pump. Expression of Pgp on the cell surface is sufficient to render cells resistant to multiple cytotoxic drugs, including many anti-cancer agents. Pgp-mediated MDR appears to be an important clinical component of tumor resistance in tumors of different types, and mdr1 gene expression correlates with resistance to chemotherapy in different types of cancer.
Sequence analysis of the mdr1 gene indicates that Pgp consists of 1280 amino acids distributed between two homologous (43% identity) halves..sup.1 Each half of the molecule has six hydrophobic transmembrane domains and each has an ATP binding site within the large cytoplasmic loops. Only about 8% of the molecule is extracellular, and the carbohydrate moiety (approximately 30 kDa) is bound to sites in this region.
With the advent of knowledge about the central role in MDR played by Pgp, agents with a potential for reversing MDR have been targeted at Pgp. Several classes of drugs, including calcium channel blockers, e.g., verapamil, immunosuppresants such as cyclosporines and steroid hormones, calmodulin inhibitors and several other compounds, were found to enhance the intracellular accumulation and cytotoxic action of Pgp-transported drugs..sup.2 Many of these agents were found to inhibit drug binding or transport by Pgp..sup.3 Some of these agents themselves were found to bind to and be effluxed by Pgp.sup.4, suggesting that their enhancing effects on the cytotoxicity of Pgp substrates are due, at least in part, to competition for drug binding sites on this protein, rather than to effects on its function.
Certain of these agents may have additional intracellular pleiotropic effects in MDR cells that may limit their applicability as specific inhibitors of the efflux pump action of Pgp. Furthermore, most of the known MDR-reversing drugs used in clinical trials have major side effects unrelated to inhibition of Pgp, such as calcium channel blockage (verapamil) or immunosuppression (cyclosporines and steroids), which restricts their clinically achievable dosage.
The use of anti-Pgp antibodies to circumvent Pgp-MDR offers the prospect of specificity, as the antibodies should target only Pgp, and the only toxicity should be that potentially arising from the administration of a protein. Furthermore, antibody binding is likely to have a more-prolonged inhibitory effect than would transient binding of a competitive inhibitor.
Only antibodies that react with an extracellular epitope of Pgp would be able to react with the efflux pump protein in the plasma membrane of intact cells and potentially influence, i.e. reverse, MDR. Antibodies directed to the cytoplasmic portion of Pgp, such as C2195.sup.5, are unlikely to be useful for reversal of MDR.
Monoclonal antibodies ("mAb"), termed MRK-16 and MRK-17, were produced by immunizing mice with doxorubicin-resistant K-562 human leukemia cells; both antibodies recognized pgp..sup.6 MRK-16 mAb modulated vincristine and actinomycin D transport in resistant cells, and MRK-17 mAb specifically inhibited the growth of the resistant cells. MRK-16 mAb increased the in vivo toxicity of vincristine to a human MDR cell line (colon cancer) grown as a xenograft in nude mice..sup.7 The in vitro potentiation of drug cytotoxicity by MRK-16 mAb was, however, weak relative to known chemical inhibitors of Pgp action, and was apparently limited to only two Pgp substrates (vincristine and actinomycin D), having no effect on cytotoxicity by doxorubicin..sup.6 Treatment with MRK-16 mAb of athymic mice previously inoculated with drug resistant human ovarian cancer cells 2780.sup.AD caused regression of established subcutaneous tumors..sup.8 A recombinant chimeric antibody that combines the variable region of MRK-16 with the Fc portion of human antibodies was reported to be more effective than parent MRK-16 mAb in increasing cytotoxicity in vitro..sup. 9
Monoclonal antibodies HYB-241 and HYB-612, which recognize an external epitope of Pgp, have been reported to increase the accumulation of vincristine and actinomycin D in tumor cells and to increase the cytotoxicity of combinations of these drugs with verapamil..sup.10
A mouse IgG.sub.2a mAb, termed MAb657, has been reported to react with mdr1 gene-expressing cells and with MDR human cells..sup.11 Although this mAb was shown to increase the susceptibility of MDR cells to cytotoxicity mediated by human peripheral blood lymphocytes, it is not known to have an inhibitory effect on the drug efflux pump function of Pgp.
As will be detailed in the description of the invention below, the effects of the mAb of the present invention can be distinguished from those of MRK-16, HYB-241 and HYB-612 mAbs on many levels, including effects on inhibition of rhodamine 123 efflux from MDR cells, potentiation of the effects of vinblastine on cell growth and colony formation, potentiation of the cytotoxic effect of doxorubicin, epitopic specificity and detergent sensitivity.
An important need remains for novel monoclonal antibodies that recognize extracellular domains of human Pgp on the surface of intact cells, that exhibit strong inhibitory effects on Pgp-mediated efflux of anti-cancer drugs from human tumor cells, that reverse resistance to a wide variety of cytotoxic drugs that are transported by the human Pgp system, and that are at least as potent as commonly used chemical inhibitors of Pgp but without undesirable side effects. A hybridoma producing such a specific mAb has been produced, and the properties and uses of this antibody, as well as fragments and recombinant derivatives thereof, are described below.